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Thursday, July 25, 2013

2.Access Methods ContentionAn Access method – set of rules – how network nodes share the transmission medium.A pair of fundamental philosophies areFirst come First servedTake turnMedia Access Methods of three types Contention Polling Token PassingThe way devices on the network – physically connected is known as the topology .Topology is a map of the network – plan for how the cabling interconnect the nodes.

4.ContentionIn pure contention based control – any computer can transmit – any time.Collision occurs -- two computers – transmit – at the same time –disrupt each other’s signaling Modern Contention Based networks – devices listens for other signals on the media before transmitting – Collisions are not totally eliminated – kept down to manageable levels – technique known as Carrier Sense Multiple Access or CSMATwo types of CSMA are ………CSMA/CD Carrier Sense Multiple Access / Collision DetectionCSMA/CA Carrier Sense Multiple Access / Collision Avoidance

5.CSMA/CDDevice “listens” to media – any other transmissionMedia quiet -- device proceeds to start transmitting data After transmits its data – listens to the media to detect any collisionIf collision detects – device send a signal for all other device not to send data for a small period of time to clear all signals from the mediaTrasmitting device – wait a random amount of time b’fore sending its dataIf second collision occurs with same device – double the random time out b’fore transmit again – once transmission successfully, other devices – allowed to transmit again

10.Token PassingSpecial packet is passed from computer to computer -- TokenA computer that wants to transmit must wait for a free token.Computer takes control of the token and transmits data. Only this computer is allowed to transmit; others must wait for control of the token.Receiving computer strips the data from the token and sends an acknowledgment.Original sending computer receives the acknowledgment and sends the token on.